1. Field of the Invention
The present invention relates to a thermal infrared detecting device having a thermal isolation structure.
2. Description of the Prior Art
Examples of a thermal infrared detecting device are bolometer, pyroelectric, and thermocouple sensors. To improve their sensitivities, development of a thermal isolation structure using micromachining technique, a heat-sensitive material, and an infrared absorption film is essential.
To increase the infrared absorbance of a pyroelectric sensor, a structure shown in FIG. 1 is formed (Parsons and Pedder, J. Vac. Sci. Technol. A6 (1988) 1686). A perfect reflection film (this film also serves as an electrode) 12 is formed on a pyroelectric material 11, and an insulating film 13 having a known refractive index (n) and an absorbance of almost zero is formed to a thickness of xcex/(4n) where xcex is the wavelength of an incident infrared ray. Next, a semitransparent thin film (infrared absorption film) 14 matching the vacuum impedance (377 xcexa9/xe2x96xa1) is formed, thus completing a structure for absorbing an infrared ray. An infrared ray is absorbed by free electrons in the semi-transparent thin film due to destructive interference between the perfect reflection film 12 also serving as the electrode and the semitransparent thin film 14. The absorbed infrared ray is converted into heat to change the spontaneous polarization of the pyroelectric material, and finally read as a change in amount of surface electric charge.
FIG. 2 (U.S. Pat. No. 5,369,280 (1994)) and FIG. 3 (U.S. Pat. No. 5,286,976 (1994)) show the infrared absorption structure of a bolometer sensor. For the infrared absorption structure shown in FIG. 2, a silicon oxide film 15 and an insulating support film 16 are formed on a silicon substrate 1, and then, a lower electrode 17, a bolometer thin film 18, and an upper electrode 19 are formed as an infrared absorption film. The lower electrode is formed as a perfect reflection film. On the other hand, the upper electrode is formed very thin. The resistivity and film thickness of the upper electrode are set by selecting the electrode material and film deposition conditions such that the upper electrode matches the vacuum impedance (377 xcexa9/xe2x96xa1). As described above, letting n be the refractive index of the material of the bolometer thin film 18, and xcex be the wavelength of an incident infrared ray, the thickness of the bolometer thin film 18 is given by xcex/(4n). The infrared absorption mechanism is the same as that shown in FIG. 1: destructive interference takes place between the upper electrode and the lower electrode so that the incident infrared ray is absorbed by free electrons in the upper electrode. The absorbed infrared ray is converted into heat to change the resistance of the bolometer material, and read as a voltage change by flowing a bias current.
A bolometer thermal device shown in FIG. 3 can be formed on a flat silicon substrate 1 without etching the silicon substrate 1, unlike the sensor shown in FIG. 2. Since a large area can be ensured to form a readout circuit in manufacturing the array sensor, the fill factors of the light-receiving portion can be increased, and in other words, the sensitivity of the sensor can be improved. An insulating protective film 20, a perfect reflection film 21, and a sacrificial layer (a cavity 27 is formed therein in the post-process) are formed on the silicon substrate 1, and a silicon nitride film 22, a bolometer thin film 23, and a silicon nitride film 24 are formed on the sacrificial layer and its slanting surface. An electrode interconnection (not shown) from the bolometer thin film to the readout circuit in the silicon substrate is sandwiched between two silicon nitride films and connected to the readout circuit through a contact pad 26. A semitransparent thin film 25 is formed on the silicon nitride film 24. By etching the sacrificial layer in the resultant multilayered structure, a highly sensitive thermal infrared detecting device having a thermal isolation structure can be manufactured. This structure has the same infrared absorption mechanism as described above: destructive interference occurs between the semitransparent thin film 25 and the perfect reflection film 21, so the incident infrared ray is absorbed by free electrons in the semitransparent thin film. The absorbed infrared ray is converted into heat to change the resistance of the bolometer material, and read out as a voltage change by flowing a bias current.
As the first problem of the prior arts shown in FIGS. 1 to 3, the yield of thermal infrared devices lowers. The reason for this is as follows. The semitransparent thin film functioning as the infrared absorption film must be fabricated into a predetermined shape after formation of the film, resulting in an increase in the number of processes. In addition, generally, the semitransparent thin film must be made very thin, and its sheet resistance is largely affected by the roughness of the underlayer.
As the second problem, in the prior art shown in FIG. 2, the thermal time constant becomes large, so an afterimage is generated in imaging. The reason for this is as follows. When the infrared absorbance is to be increased in the upper electrode/bolometer thin film/lower electrode structure on the support film, the bolometer thin film must be thickened, resulting in a large thermal capacity.
The present invention has been made in consideration of the above situation, and has as its object to provide a thermal infrared detecting device having a high manufacturing yield, high productivity, small thermal time constant, and a minimum afterimage.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a thermal infrared detecting device comprising a lower layer portion having a readout circuit, and an upper layer portion having a bolometer thin film covered with an insulating protective film to perform heat/resistance conversion, wherein the upper layer portion and the lower layer portion are spaced apart from each other while sandwiching a vacuum or sealed gas to form a thermal isolation structure, and also electrically connected to each other through an electrode film formed on the insulating protective film or in the insulating protective film, and the bolometer thin film also serves as an infrared absorption film.
According to the second aspect of the present invention, the bolometer thin film of the first aspect has a resistivity of 1 to 10 mxcexa9cm and a thickness of 500 to 2,000 xc3x85.
According to the third aspect of the present invention, the bolometer thin film of the second aspect essentially consists of a manganese-based oxide.
According to the fourth aspect of the present invention, the bolometer thin film of the second aspect essentially consists of a vanadium compound.
As is apparent from the above aspects, according to the present invention, by adding the function of the infrared absorption film to the bolometer thin film of the thermal infrared detecting device, the manufacturing process becomes simple, so the yield of the device can be improved. In addition, since the thermal time constant becomes small to minimize the afterimage, a real-time infrared image without any sense of incompatibility can be obtained.
As the first effect, the manufacturing process is simplified, so the yield of the thermal infrared detecting device can be improved. The reason for this is as follows. According to the present invention, the bolometer thin film also functions as the infrared absorption film, and no semitransparent thin film need be formed, unlike the prior art.
As the second effect, the afterimage can be minimized in sensing an infrared image using the thermal infrared detecting device, so a real-time image without any sense of incompatibility can be obtained. The reason for this is as follows. According to the present invention, since the bolometer thin film also has the function of the infrared absorption film, the thermal capacity of the light-receiving portion (a portion where the bolometer thin film is formed in FIG. 4B) is decreased, and the thermal time constant also becomes small.
The above and many other advantages, features and additional objects of the present invention will become manifest to those versed in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples.